Macbook Air-Upgrading Memory, 1.4Ghz or 1.6GHz?

I want to get an 11.6" MacBook Air. I decided against the base model since I definitely want the 128GB SSD. I plan to buy an external 1TB Hard Drive to store a lot of data on but want to make sure that all of my applications and important data all fits on my internal hard drive so I don't have to always take the external drive with me.

I looked at the $1199 model which has the 128GB but it only has 2GB and has the base 1.4Ghz processor.

Is there a big speed difference between the 1.4GHz and 1.6GHz option?
I have seen an option where I can buy it for $1399 and it gives me the 1.6GHz CPU and 4GB of memory.

I do a bit of multitasking and most of what would be done on it is web browsing, email, office (excel, word), Adobe Acrobat, Filemaker, and Bento.
For this use would 4GB be better than 2GB and would the 1.6GHz CPU help a lot?

One last question. I have often purchased my MacBook Pro's with the least memory available and purchased memory from OWC. Is it possible to upgrade the memory in the MacBook Air after purchase?

As Crimsom advises, buy the "Air" with the most memory. The Air has no user serviceable parts or upgrades you can deal with. What you buy is what you get, so buy the best Air you can afford now. And by the way, just so you know - the Air does not have a SSD, it's Flash memory which is quite different.